improve description of ChainerMN requirements
improve description of ChainerMN requirements

chainer/chainer,wkentaro/chainer,pfnet/chainer,niboshi/chainer,chainer/chainer,okuta/chainer,niboshi/chainer,keisuke-umezawa/chainer,okuta/chainer,tkerola/chainer,okuta/chainer,keisuke-umezawa/chainer,wkentaro/chainer,hvy/chainer,keisuke-umezawa/chainer,hvy/chainer,jnishi/chainer,jnishi/chainer,jnishi/chainer,chainer/chainer,chainer/chainer,jnishi/chainer,hvy/chainer,niboshi/chainer,niboshi/chainer,okuta/chainer,wkentaro/chainer,wkentaro/chainer,hvy/chainer,keisuke-umezawa/chainer